Part API changes

- Allow filtering parts with null parent (top-level category parts)
- Option to include sub-category parts or not

InvenTree/part/api.py

@@ -243,12 +243,25 @@ class PartList(generics.ListCreateAPIView):
         # Start with all objects
         parts_list = Part.objects.all()
 
-        if cat_id:
-            try:
-                category = PartCategory.objects.get(pk=cat_id)
-                parts_list = parts_list.filter(category__in=category.getUniqueChildren())
-            except PartCategory.DoesNotExist:
-                pass
+        cascade = str2bool(self.request.query_params.get('cascade', False))
+
+        if cat_id is not None:
+
+            if isNull(cat_id):
+                parts_list = parts_list.filter(category=None)
+            else:
+                try:
+                    cat_id = int(cat_id)
+                    category = PartCategory.objects.get(pk=cat_id)
+
+                    # If '?cascade=true' then include parts which exist in sub-categories
+                    if cascade:
+                        parts_list = parts_list.filter(category__in=category.getUniqueChildren())
+                    # Just return parts directly in the requested category
+                    else:
+                        parts_list = parts_list.filter(category=cat_id)
+                except (ValueError, PartCategory.DoesNotExist):
+                    pass
 
         # Ensure that related models are pre-loaded to reduce DB trips
         parts_list = self.get_serializer_class().setup_eager_loading(parts_list)